Hydrophilic pressure sensitive adhesives are used in the medical field for a variety of skin-contacting applications as detailed in "An Overview of Skin Contact Applications for Pressure-sensitive Adhesives", D. G. Pierson, Tappi Journal, 73 (6), 101-107 (June, 1990). These applications include use as adhesives which absorb exudate from wounds, as ionically conductive adhesives in biomedical electrodes, and in iontophoretic (electrically enhanced) transdermal delivery of polar drugs.
Hydrophilic pressure sensitive adhesives are networks of a polar polymer, generally crosslinked and plasticized with a compatible humectant material such as glycerin or poly(ethylene glycol). Water, a salt, or a drug may also be included depending on the application. A relatively thick layer of adhesive is typically used (0.25 to 2.5 mm (10 to 100 mil) or greater).
Several approaches have been developed to prepare these hydrophilic pressure sensitive adhesives. They can be generated by delivering a moderate viscosity solution from solvent or water and drying (exemplified by U.S. Pat. No. 5,276,079); by extrusion coating a high viscosity precursor (exemplified by U.S. Pat. Nos. 4,674,512 and 4,593,053); by polymerization on-web of a low viscosity mixture of monomer, crosslinker, and humectant (exemplified by U.S. Pat. Nos. 4,524,087; 4,539,996; 4,554,924; and 4,848,353); and by crosslinking on-web a moderate viscosity precursor of polymer and humectant. For this final approach the crosslinking can be brought about by exposure of a non-functional polymer to high energy irradiation (exemplified by U.S. Pat. Nos. 4,699,146 and 4,750,482; and PCT Publication WO 93/10163), by mixing in a reactive crosslinking agent during the coating process (exemplified by U.S. Pat. Nos. 4,515,162 and 5,160,328; and PCT Publication WO 93/22380or by exposure of a vinyl functional polymer to a free radical source. Such vinyl functional polymers can be prepared by reaction of amine terminated poly(ethylene oxide) with, for example, vinyl dimethyl azlactone (e.g., PCT Publication WO 94/12585), reaction of isocyanate functional polyoxyalkylene oligomer with hydroxyethyl methacrylate (e.g., EPO Publication EP 271,292), and reaction of poly(vinyl ether/maleic anhydride) with allyl amine or allyl alcohol (e.g., British Publication GB 2,268,495). These approaches are applicable to a relatively narrow range of polar polymers and may require the use of an aprotic solvent during synthesis which must be removed.